1. Field of the Invention
This invention relates to a semiconductor laser device, especially, an internal-reflection-interference semiconductor laser device, which has a stabilized oscillation wavelength over a wide range of temperatures.
2. Background of the Invention
Semiconductor laser devices which are being mass-produced can oscillate at a low threshold current level, and their various characteristics, such as the single transverse mode, the single longitudinal mode, and life span have satisfactory qualities. However, problems remain to be solved with regard to the stability of the oscillation wavelength in longitudinal modes. With changes in temperature or in the driving current, the oscillation wavelength changes continuously or discontinuously, and moreover, optical output power noise is generated. In particular, this noise is accentuated when the laser devices are irradiated with light from the outside, and/or laser light output is reflected by external optical devices.
In order to solve these problems, internal-reflection-interference semiconductor laser devices have been proposed, an example of which is shown in FIG. 5. This laser device has an n-substrate 1, an n-cladding layer 2, an active layer 3, a p-cladding layer 4, a p-cap layer 5, and a current-blocking oxide film 6. The reference 7 is an internal reflecting section, in which the portion of the active layer 3 (or the cladding layer 2) corresponding to the channel 11, which is constructed to be perpendicular to the laser oscillation direction is different in thickness from the other portion of the active layer 3 (or the cladding layer 2), so that this reflecting section 7 attains an internal reflection effect. The resulting wafer is separated by the reflecting section 7 into two laser operation areas 8 and 9, one (i.e., a first operation area 8) having the internal-cavity length l.sub.1 and the other (i.e., a second operation area 9) having the internal-cavity length l.sub.2. Due to the interference effect of lights attained between the laser operation areas 8 and 9, stability of the oscillating wavelength in longitudinal modes can be expected.
The space .lambda..sub.1 between the longitudinal modes of the first laser operation area 8 is proportional to .lambda..sup.2 /2nl.sub.1, and the space .DELTA..lambda..sub.2 between the longitudinal modes of the second laser operation area 9 is proportional to .lambda..sup.2 /2nl.sub.2, wherein .lambda. is the oscillation wavelength and n is the refractive index of the active layer. Moreover, due to the interference of the longitudinal modes between the first laser operation area 8 and the second laser operation area 9, a broad space .DELTA.(=.lambda..sup.2 /2n.vertline.l.sub.2 -l.sub.1 .vertline.) between the longitudinal modes is created so that a stabilized oscillation can be attained in only the longitudinal mode around the peak of the gain distribution.
However, with an internal-reflection-interference semiconductor laser device mentioned above, it is difficult to obtain a strong reflection from the internal reflecting area 7. Therefore, because the selectivity for the wavelength is poor, it is not possible to stabilize the longitudinal mode over a wide range of temperatures. Thus, in practice, the oscillation wavelength is stabilized over a temperature difference of 5.degree.-10.degree. C. at the most. Moreover, it is impossible to suppress completely the instability of a longitudinal mode arising from reflected light.